As a conventional liquid crystal panel setting structure, a structure in which a liquid crystal panel is directly secured to a photosynthetic prism is disclosed in the official gazettes of Japanese Patent Application Laid-Open No. 4-10128 and Japanese Patent Application No. 4-270557. In these official gazettes, it is particularly shown that an imaging optical system is decreased in size, weight, and number of parts and the cost is decreased due to reduction of assembling and controlling operations by omitting a mechanism for performing picture-element alignment control (hereafter referred to as alignment control) between a plurality of liquid crystal panels (also known as light bulbs) for modulating the light separated into red, green, and blue and arrangement control (hereafter focus control) of each liquid crystal layer surface serving as an object within the allowable focal depth of a projection lens.
In the case of the structure disclosed in these official gazettes, however, because a liquid crystal panel is firmly secured to a photosynthetic prism, it is very troublesome to remove the liquid crystal panel if repair or reproduction is necessary. Repair of a product is not frequently required after the product is shipped from a factory to a market. In a factory before shipping a product, however, repair and reproduction are frequently required due to the following causes.
(1) Replacement of a defective product with a non-defective product because a pattern is disconnected due to chipping, a circuit device is broken down due to static electricity incoming from an electrode terminal, or contamination is caused by hand oil or adhesive.
(2) Replacement of a defective product with a non-defective product when a claim suddenly occurs because each manufacturing process is not stabilized in the beginning of mass production of a liquid crystal panel.
Moreover, decreasing the alignment control accuracy to 1/2 picture elements or less between picture elements is indispensable to reduce the unclearness of a screen and prevent color drift. When considering the magnification chromatic aberration of a projection lens and the accuracy deviation of the synthetic surface of a photosynthetic prism, it is necessary to keep the control accuracy within several microns. Moreover, though the focus control accuracy depends on a set value of the allowable scattering circle of a projected image and a set F value, the allowable focal depth comes to approx. 100 .mu.m and thus, control is necessary in this range. Therefore, an exclusive expensive regulator capable of adjusting six axial directions is necessary. Furthermore, because it is impossible to set the expensive regulator to every foothold for performing after-sale services, a product must be repaired or reproduced by returning it to a limited foothold or a manufacturing factory. Therefore, the following problems are pointed out.
(1) After-sale service is interrupted or a user must pay much money when replacing a defective imaging optical-system unit with a new one.
(2) Also when performing repair or reproduction in a manufacturing factory, the serviceability ratio in mass production is lowered and resultingly, the cost is increased.
Moreover, a liquid crystal panel and a photosynthetic prism are expensive parts. Therefore, when three high-precision liquid crystal panels for red, blue, and green capable of corresponding to the 640.times.480 dot arrangement are used, it is very wasteful to do away with them as defective products because they account for 20 to 40% of the part cost and the cost increases.